High-strength steel sheet and production method for same, and production method for high-strength galvanized steel sheet

ABSTRACT

Disclosed is a high-strength steel sheet having a predetermined chemical composition and a steel microstructure that contains, by area, 25-80% of ferrite and bainitic ferrite in total, and 3-20% of martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 μm or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, an area ratio of retained austenite having a mean C content in mass % at least 2.1 times the C content in the steel sheet in mass % is 60% or more of an area ratio of the entire retained austenite.

TECHNICAL FIELD

This disclosure relates to a high-strength steel sheet with excellentformability which is mainly suitable for automobile structural membersand a method for manufacturing the same, and in particular, to provisionof a high-strength steel sheet that has a tensile strength (TS) of 780MPa or more and that is excellent not only in ductility, but also instretch flangeability and stability as a material.

BACKGROUND

In order to secure passenger safety upon collision and to improve fuelefficiency by reducing the weight of automotive bodies, high-strengthsteel sheets having a tensile strength (TS) of 780 MPa or more, andreduced in thickness, have been increasingly applied to automobilestructural members. Further, in recent years, examination has been madeof applications of ultra-high-strength steel sheets with 980 MPa and1180 MPa grade TS.

In general, however, strengthening of steel sheets leads todeterioration in formability. It is thus difficult to achieve bothincreased strength and excellent formability. Therefore, it is desirableto develop steel sheets with increased strength and excellentformability.

In addition, strengthening of steel sheets and reducing the thicknesssignificantly deteriorates the shape fixability of the steel sheets. Toaddress this problem, a press mold design is widely used that takes intoconsideration the amount of geometric change after release from thepress mold as predicted at the time of press forming.

However, the amount of geometric change is predicted on the basis of TS,and accordingly increased variation in TS of steel sheets results in thepredicted value of geometric change deviating more markedly from themeasured amount of geometric change, inducing malformation. Such steelsheets suffering malformation require adjustments after subjection topress forming, such as sheet metal working on individual steel sheets,significantly decreasing mass production efficiency. Accordingly, thereis a demand for minimizing variation in TS of steel sheets.

To meet this demand, for example, JP2004218025A (PTL 1) describes ahigh-strength steel sheet with excellent workability and shapefixability comprising: a chemical composition containing, in mass %, C:0.06% or more and 0.60% or less, Si+Al: 0.5% or more and 3.0% or less,Mn: 0.5% or more and 3.0% or less, P: 0.15% or less, and S: 0.02% orless; and a microstructure that contains tempered martensite: 15% ormore by area to the entire microstructure, ferrite: 5% or more and 60%or less by area to the entire microstructure, and retained austenite: 5%or more by volume to the entire microstructure, and that may containbainite and/or martensite, wherein a ratio of the retained austenitetransforming to martensite upon application of a 2% strain is 20% to50%.

JP2011195956A (PTL 2) describes a high-strength thin steel sheet withexcellent elongation and hole expansion formability, comprising: achemical composition containing, in mass %, C: 0.05% or more and 0.35%or less, Si: 0.05% or more and 2.0% or less, Mn: 0.8% or more and 3.0%or less, P: 0.0010% or more and 0.1000% or less, S: 0.0005% or more and0.0500% or less, and Al: 0.01% or more and 2.00% or less, and thebalance consisting of iron and incidental impurities; and ametallographic structure that includes a dominant phase of ferrite,bainite, or tempered martensite, and retained austenite in an amount of3% or more and 30% or less, wherein at a phase interface at which theaustenite comes in contact with ferrite, bainite, and martensite,austenite grains that satisfy Cgb/Cgc>1.3 are present in an amount of50% or more, where Cgc is a central carbon concentration and Cgb is acarbon concentration at grain boundaries of austenite grains.

JP201090475A (PTL 3) describes “a high-strength steel sheet comprising achemical composition containing, in mass %, C: more than 0.17% and 0.73%or less, Si: 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.1% orless, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less, whereSi+Al is 0.7% or more, and the balance consisting of Fe and incidentalimpurities; and a microstructure that contains martensite: 10% or moreand 90% or less by area to the entire steel sheet microstructure,retained austenite content: 5% or more and 50% or less, and bainiticferrite in upper bainite: 5% or more by area to the entire steel sheetmicrostructure, wherein the steel sheet satisfies conditions that 25% ormore of the martensite is tempered martensite, a total of the area ratioof the martensite to the entire steel sheet microstructure, the retainedaustenite content, and the area ratio of the bainitic ferrite in upperbainite to the entire steel sheet microstructure is 65% or more, and anarea ratio of polygonal ferrite to the entire steel sheet microstructureis 10% or less, and wherein the steel sheet has a mean carbonconcentration of 0.70% or more in the retained austenite and has atensile strength (TS) of 980 MPa or more.

JP2008174802A (PTL 4) describes a high-strength cold-rolled steel sheetwith a high yield ratio and having a tensile strength of 980 MPa ormore, the steel sheet comprising, on average, a chemical compositionthat contains, by mass %, C: more than 0.06% and 0.24% or less, Si: 0.3%or less, Mn: 0.5% or more and 2.0% or less, P 0.06% or less, S: 0.005%or less, Al: 0.06% or less, N 0.006% or less, Mo: 0.05% or more and0.50% or less, Ti: 0.03% or more and 0.2% or less, and V: more than0.15% and 1.2% or less, and the balance consisting of Fe and incidentalimpurities, wherein the contents of C, Ti, Mo, and V satisfy0.8≦(C/12)/{(Ti/48)+(Mo/96)+(V/51)}≦1.5, and wherein an area ratio offerrite phase is 95% or more, and carbides containing Ti, Mo, and V witha mean grain size of less than 10 nm are diffused and precipitated,where Ti, Mo, and V contents represented by atomic percentage satisfyV/(Ti+Mo+V)≧0.3.

JP2010275627A (PTL 5) describes a high-strength steel sheet withexcellent workability comprising a chemical composition containing, inmass %, C: 0.05% or more and 0.30% or less, Si: 0.01% or more and 2.50%or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and0.100%, S: 0.02% or less, and Al: 0.010% to 1.500%, where Si+Al: 0.5% to3.0%, and the balance consisting of Fe and incidental impurities; and ametallic structure that contains, by area, ferrite: 20% or more,tempered martensite: 10% or more and 60% or less, and martensite: 0% to10%, and that contains, by volume, retained austenite: 3% to 10%, wherea ratio m/f of a Vickers hardness (m) of the tempered martensite to aVickers hardness (f) of the ferrite is 3.0 or less.

JP201132549A (PTL 6) describes a high-strength hot-dip galvanized steelstrip that is excellent in formability and that is reduced in materialproperty variation in the steel strip, the steel sheet comprising achemical composition containing, in mass %, C: 0.05% or more and 0.2% orless, Si: 0.5% or more and 2.5% or less, Mn: 1.5% or more and 3.0% orless, P: 0.001% or more and 0.05% or less, S: 0.0001% or more and 0.01%or less, Al: 0.001% or more and 0.1% or less, and N: 0.0005% or more and0.01% or less, and the balance consisting of Fe and incidentalimpurities; and a microstructure that contains ferrite and martensite,wherein the ferrite phase accounts for 50% or more by area of the entiremicrostructure and the martensite accounts for 30% or more and 50% orless by area of the entire microstructure, and wherein the differencebetween the highest tensile strength and the lowest tensile strength is60 MPa or less in the steel strip.

CITATION LIST Patent Literature

PTL 1: JP2004218025A

PTL 2: JP2011195956A

PTL 3: JP201090475A

PTL 4: JP2008174802A

PTL 5: JP2010275627A

PTL 6: JP201132549A

SUMMARY Technical Problem

However, although PTL 1 teaches the high-strength steel sheet isexcellent in workability and shape fixability, PTL 2 teaches thehigh-strength thin steel sheet is excellent in elongation and holeexpansion formability, and PTL 3 teaches the high-strength steel sheetis excellent in workability, in particular ductility and stretchflangeability, none of these documents consider the stability of thesteel sheet as a material, namely variation of TS.

The high-strength cold-rolled steel sheet with a high yield ratiodescribed in PTL 4 uses expensive elements, Mo and V, which results inincreased costs. Further, the steel sheet has a low elongation (EL) aslow as approximately 19%.

The high-strength steel sheet described in PTL 5 exhibits, for example,TS×EL of approximately 24000 MPa·% with a TS of 980 MPa or more, whichremain, although may be relatively high when compared to general-usematerial, insufficient in terms of elongation (EL) to meet the ongoingrequirements for steel sheets.

While PTL 6 teaches a technique for providing a high-strength hot-dipgalvanizing steel strip that is reduced in material property variationin the steel strip and is excellent in formability, this technique doesnot make use of retained austenite, and the problem of low EL remains tobe solved.

It could thus be helpful to provide a high-strength steel sheet that hasa tensile strength (TS) of 780 MPa or more and that is excellent notonly in ductility, but also in stretch flangeability and stability as amaterial, and a production method therefor. As used herein, “excellentin stability as a material” refers to a case where ΔTS, which is theamount of variation of TS upon the annealing temperature duringannealing treatment changing by 40° C. (±20° C.), is 40 MPa or less(preferably 36 MPa or less), and ΔEL, which is the amount of variationof EL upon the annealing temperature changing by 40° C., is 3% or less(preferably 2.4% or less).

Solution to Problem

As a result of intensive studies made to solve the above problems, wediscovered the following.

A slab is heated to a predetermined temperature, and subjected to hotrolling to obtain a hot-rolled sheet. After the hot rolling, thehot-rolled sheet is optionally subjected to heat treatment forsoftening. The hot-rolled sheet is then subjected to cold rolling,followed by first annealing treatment at an austenite single phaseregion, and subsequent cooling rate control to suppress ferritetransformation and pearlite transformation.

Subsequently, a single phase of martensite, a single phase of bainite,or a mixed phase of martensite and bainite is caused to be dominantlypresent in the microstructure of the steel sheet before subjection tosecond annealing, and as a result, non-polygonal ferrite and bainiticferrite are produced in large amounts during the cooling and retainingprocess after the second annealing.

The large amounts of non-polygonal ferrite and bainitic ferrite thusproduced may ensure the formation of proper amounts of fine retainedaustenite. This enables the provision of a microstructure in whichferrite and bainitic ferrite are dominantly present and which containsfine retained austenite, and thus the production of a high-strengthsteel sheet that has a TS of 780 MPa or more and that is excellent notonly in ductility, but also in stretch flangeability and stability as amaterial.

This disclosure has been made based on these discoveries.

Specifically, the primary features of this disclosure are as describedbelow.

1. A high-strength steel sheet comprising: a chemical compositioncontaining (consisting of), in mass %, C: 0.08% or more and 0.35% orless, Si: 0.50% or more and 2.50% or less, Mn: 1.60% or more and 3.00%or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and thebalance consisting of Fe and incidental impurities; a steelmicrostructure that contains, by area, 25% or more and 80% or less offerrite and bainitic ferrite in total, and 3% or more and 20% or less ofmartensite, and that contains, by volume, 10% or more of retainedaustenite, wherein the retained austenite has a mean grain size of 2 μmor less, a mean Mn content in the retained austenite in mass % is atleast 1.2 times the Mn content in the steel sheet in mass %, and an arearatio of retained austenite having a mean C content in mass % at least2.1 times the C content in the steel sheet in mass % is 60% or more ofan area ratio of the entire retained austenite.

2. The high-strength steel sheet according to 1., wherein the chemicalcomposition further contains, in mass %, at least one element selectedfrom the group consisting of Al: 0.01% or more and 1.00% or less, Ti:0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% orless, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00%or less, Sb: 0.0020% or more and 0.2000% or less, Sn: 0.0020% or moreand 0.2000% or less, Ta: 0.0010% or more and 0.1000% or less, Ca:0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and 0.0050% orless, and REM: 0.0003% or more and 0.0050% or less.

3. A production method for a high-strength steel sheet, the methodcomprising: heating a steel slab having the chemical composition asrecited in 1. or 2. to 1100° C. or higher and 1300° C. or lower; hotrolling the steel slab with a finisher delivery temperature of 800° C.or higher and 1000° C. or lower to obtain a steel sheet; coiling thesteel sheet at a mean coiling temperature of 450° C. or higher and 700°C. or lower; subjecting the steel sheet to pickling treatment;optionally, retaining the steel sheet at a temperature of 450° C. orhigher and Ac₁ transformation temperature or lower for 900 s or more and36000 s or less; cold rolling the steel sheet at a rolling reduction of30% or more; subjecting the steel sheet to first annealing treatmentwhereby the steel sheet is heated to a temperature of 820° C. or higherand 950° C. or lower; cooling the steel sheet to a first cooling stoptemperature at or below Ms at a mean cooling rate to 500° C. of 15° C./sor higher; subjecting the steel sheet to second annealing treatmentwhereby the steel sheet is reheated to a temperature of 740° C. orhigher and 840° C. or lower; cooling the steel sheet to a temperature ina second cooling stop temperature range of 300° C. to 550° C. at a meancooling rate of 10° C./s or higher and 50° C./s or lower; and retainingthe steel sheet at the second cooling stop temperature range for 10 s ormore, to produce the high-strength steel sheet as recited in 1. or 2.

4. The production method for a high-strength steel sheet according to3., the method further comprising after the retaining at the secondcooling stop temperature range, subjecting the steel sheet to thirdannealing treatment whereby the steel sheet is heated to a temperatureof 100° C. or higher and 300° C. or lower.

5. A production method for a high-strength galvanized steel sheet, themethod comprising subjecting the high-strength steel sheet as recitedin 1. or 2. to galvanizing treatment.

Advantageous Effect

According to the disclosure, it becomes possible to effectively producea high-strength steel sheet that has a TS of 780 MPa or more, and thatis excellent not only in ductility, but also in stretch flangeabilityand stability as a material. Also, a high-strength steel sheet producedby the method according to the disclosure is highly beneficial inindustrial terms, because it can improve fuel efficiency when appliedto, e.g., automobile structural members by a reduction in the weight ofautomotive bodies.

DETAILED DESCRIPTION

The following describes one of the embodiments according to thedisclosure.

According to the disclosure, a slab is heated to a predeterminedtemperature and hot-rolled to obtain a hot-rolled sheet. After the hotrolling, optionally, the hot-rolled sheet is subjected to heat treatmentfor softening. The hot-rolled sheet is then subjected to cold rolling,followed by first annealing treatment at an austenite single phaseregion, after which cooling rate control is performed to suppressferrite transformation and pearlite transformation. As a result of thecooling rate control, and before subjection to second annealing, thesteel sheet has a steel microstructure in which a single phase ofmartensite, a single phase of bainite, or a mixed phase of martensiteand bainite is dominantly present. With the microstructure thusobtained, ferrite and bainitic ferrite can be produced in large amountsduring the cooling and retaining process after second annealing.Further, a proper amount of fine retained austenite can be contained inthe microstructure. A high-strength steel sheet with such microstructurecontaining fine retained austenite in which ferrite and bainitic ferriteare dominantly present has a TS of 780 MPa or more, and is excellent notonly in ductility, but also in stretch flangeability and stability as amaterial.

As used herein, “ferrite” is mainly composed of acicular ferrite whenreferring to it simply as “ferrite” as in this embodiment, yet mayinclude polygonal ferrite and/or non-recrystallized ferrite. To ensuregood ductility, however, the area ratio of non-recrystallized ferrite tosaid ferrite is preferably limited to less than 5%.

Firstly, the following explains appropriate compositional ranges forsteel according to the disclosure and the reasons for the limitationsplaced thereon.

C: 0.08 Mass % or More and 0.35 Mass % or Less

C is an element that is important for increasing the strength of steel,and has a high solid solution strengthening ability. When martensite isused for structural strengthening, C is essential for adjusting the arearatio and hardness of martensite.

When the C content is below 0.08 mass %, the area ratio of martensitedoes not increase as required for hardening of martensite, and the steelsheet does not have a sufficient strength. If the C content exceeds 0.35mass %, however, the steel sheet may be made brittle or susceptible todelayed fracture.

Therefore, the C content is 0.08 mass % or more and 0.35 mass % or less,preferably 0.12 mass % or more and 0.30 mass % or less, and morepreferably 0.17 mass % or more and 0.26 mass % or less.

Si: 0.50 Mass % or More and 2.50 Mass % or Less

Si is an element useful for suppressing formation of carbides resultingfrom decomposition of retained austenite. Si also exhibits a high solidsolution strengthening ability in ferrite, and has the property ofpurifying ferrite by facilitating solute C diffusion from ferrite toaustenite to improve the ductility of the steel sheet. Additionally, Sidissolved in ferrite improves strain hardenability and increases theductility of ferrite itself. Such Si may also reduce variation of TS andEL. To obtain this effect, the Si content needs to be 0.50 mass % ormore.

If the Si content exceeds 2.50 mass %, however, an abnormalmicrostructure develops, degrading the ductility of the steel sheet andthe stability as a material. Therefore, the Si content is 0.50 mass % ormore and 2.50 mass % or less, preferably 0.80 mass % or more and 2.00mass % or less, and more preferably 1.20 mass % or more and 1.80 mass %or less.

Mn: 1.60 Mass % or More and 3.00 Mass % or Less

Mn is effective in guaranteeing the strength of the steel sheet. Mn alsoimproves hardenability to facilitate formation of a multi-phasemicrostructure. Furthermore, Mn has the effect of suppressing formationof pearlite and bainite during a cooling process and facilitatingaustenite to martensite transformation. To obtain this effect, the Mncontent needs to be 1.60 mass % or more.

If the Mn content exceeds 3.00 mass %, however, Mn segregation becomessignificant in the sheet thickness direction, leading to deteriorationof the stability of the steel sheet as a material. Therefore, the Mncontent is 1.60 mass % or more and 3.00 mass % or less, preferably 1.60mass % or more and less than 2.5 mass %, and more preferably 1.80 mass %or more and 2.40 mass % or less.

P: 0.001 Mass % or More and 0.100 Mass % or Less

P is an element that has a solid solution strengthening effect and canbe added depending on a desired strength. P also facilitates ferritetransformation, and thus is an element effective in forming amulti-phase microstructure. To obtain this effect, the P content needsto be 0.001 mass % or more.

If the P content exceeds 0.100 mass %, however, weldability degrades. Inaddition, when a galvanized layer is subjected to alloying treatment,the alloying rate decreases, impairing galvanizing quality. Therefore,the P content is 0.001 mass % or more and 0.100 mass % or less, andpreferably 0.005 mass % or more and 0.050 mass % or less.

S: 0.0001 Mass % or More and 0.0200 Mass % or Less

S segregates to grain boundaries, makes the steel brittle during hotworking, and forms sulfides to reduce local deformability. Thus, the Scontent in steel needs to be 0.0200 mass % or less.

Under manufacturing constraints, however, the S content is necessarily0.0001 mass % or more. Therefore, the S content is 0.0001 mass % or moreand 0.0200 mass % or less, and preferably 0.0001 mass % or more and0.0050 mass % or less.

N: 0.0005 Mass % or More and 0.0100 Mass % or Less

N is an element that deteriorates the anti-aging property of steel.Smaller N contents are more preferable since deterioration of theanti-aging property becomes more pronounced particularly when the Ncontent exceeds 0.0100 mass %.

Under manufacturing constraints, however, the N content is necessarily0.0005 mass % or more. Therefore, the N content is 0.0005 mass % or moreand 0.0100 mass % or less, and preferably 0.0005 mass % or more and0.0070 mass % or less.

In addition to the above components, at least one element selected fromthe group consisting of the following may also be included: Al: 0.01mass % or more and 1.00 mass % or less, Ti: 0.005 mass % or more and0.100 mass % or less, Nb: 0.005 mass % or more and 0.100 mass % or less,Cr: 0.05 mass % or more and 1.00 mass % or less, Cu: 0.05 mass % or moreand 1.00 mass % or less, Sb: 0.0020 mass % or more and 0.2000 mass % orless, Sn: 0.0020 mass % or more and 0.2000 mass % or less, Ta: 0.0010mass % or more and 0.1000 mass % or less, Ca: 0.0003 mass % or more and0.0050 mass % or less, Mg: 0.0003 mass % or more and 0.0050 mass % orless, and REM: 0.0003 mass % or more and 0.0050 mass % or less, eitheralone or in combination. The remainder other than the aforementionedelements, of the chemical composition of the steel sheet, is Fe andincidental impurities.

Al: 0.01 Mass % or More and 1.00 Mass % or Less

Al is an element effective in forming ferrite and improving the balancebetween strength and ductility. To obtain this effect, the Al content is0.01 mass % or more. If the Al content exceeds 1.00 mass %, however,surface characteristics deteriorate. Therefore, the Al content ispreferably 0.01 mass % or more and 1.00 mass % or less, and morepreferably 0.03 mass % or more and 0.50 mass % or less.

Ti and Nb each form fine precipitates during hot rolling or annealingand increase strength. To obtain this effect, the Ti and Nb contentseach need to be 0.005 mass % or more. If the Ti and Nb contents bothexceed 0.100 mass %, formability deteriorates. Therefore, when Ti and Nbare added to steel, respective contents are 0.005 mass % or more and0.100 mass % or less.

Cr and Cu not only serve as solid-solution-strengthening elements, butalso act to stabilize austenite in a cooling process during annealing,facilitating formation of a multi-phase microstructure. To obtain thiseffect, the Cr and Cu contents each need to be 0.05 mass % or more. Ifthe Cr and Cu contents both exceed 1.00 mass %, the formability of thesteel sheet degrade. Therefore, when Cr and Cu are added to steel,respective contents are 0.05 mass % or more and 1.00 mass % or less.

Sb and Sn may be added as necessary for suppressing decarbonization of aregion extending from the surface layer of the steel sheet to a depth ofabout several tens of micrometers, which is caused by nitriding and/oroxidation of the steel sheet surface. Suppressing such nitriding oroxidation in the steel sheet surface is effective in preventing areduction in the amount of martensite formed in the steel sheet surface,and guaranteeing the strength of the steel sheet and the stability as amaterial. However, excessively adding these elements beyond 0.2000 mass% reduces toughness. Therefore, when Sb and Sn are added to steel,respective contents are 0.0020 mass % or more and 0.2000 mass % or less.

As is the case with Ti and Nb, Ta forms alloy carbides or alloycarbonitrides, and contributes to increasing the strength of steel. Itis also believed that Ta has the effect of significantly suppressingcoarsening of precipitates when partially dissolved in Nb carbides or Nbcarbonitrides to form complex precipitates, such as (Nb, Ta) (C, N), andthe suppression of coarsening of precipitates serves a stablecontribution to increasing the strength of the steel sheet. Therefore,Ta is preferably added to steel. The above-described precipitatestabilizing effect is obtained when the Ta content is 0.0010 mass % ormore. However, excessively adding Ta does not increase this effect, butinstead the alloying cost ends up increasing. Therefore, when Ta isadded to steel, the content thereof is in a range of 0.0010 mass % to0.1000 mass %.

Ca, Mg, and REM are elements used for deoxidation. These elements arealso effective in causing spheroidization of sulfides and mitigating theadverse effect of sulfides on local ductility and stretch flangeability.To obtain this effect, Ca, Mg, and REM each need to be added to steel inan amount of 0.0003 mass % or more. However, excessively adding Ca, Mg,and REM beyond 0.0050 mass % leads to increased inclusions and the like,causing defects on the steel sheet surface and internal defects.

Therefore, when Ca, Mg, and REM are added to steel, respective contentsare 0.0003 mass % or more and 0.0050 mass % or less.

The following provides a description of the microstructure.

Total Area Ratio of Ferrite and Bainitic Ferrite: 25% or More and 80% orLess

The high-strength steel sheet according to the disclosure comprises amulti-phase microstructure in which retained austenite having aninfluence mainly on ductility and martensite affecting strength arediffused in a microstructure in which soft ferrite with high ductilityis dominantly present. Additionally, to ensure sufficient ductility andstretch flangeability according to the disclosure, the total area ratioof ferrite and bainitic ferrite needs to be 25% or more. On the otherhand, to ensure the strength of the steel sheet, the total area ratio offerrite and bainitic ferrite needs to be 80% or less.

As used herein, the term “bainitic ferrite” means such ferrite that isproduced during the process of annealing at a temperature range of 740°C. to 840° C., followed by cooling to and retaining at a temperature of600° C. or lower, and that has a high dislocation density as compared tonormal ferrite.

In addition, “the area ratio of ferrite and bainitic ferrite” iscalculated with the following method. Firstly, polish a cross section ofthe steel sheet taken in the sheet thickness direction to be parallel tothe rolling direction (L-cross section), etch the cross section with 3vol. % nital, and observe ten locations at 2000 times magnificationunder an SEM (scanning electron microscope) at a position of sheetthickness×¼ (a position at a depth of one-fourth of the sheet thicknessfrom the steel sheet surface). Then, using the structure micrographsimaged with the SEM, calculate the area ratios of respective phases(ferrite and bainitic ferrite) for the ten locations with Image-Pro,available from Media Cybernetics, Inc. Then, average the results, anduse the average as “the area ratio of ferrite and bainitic ferrite.” Inthe structure micrographs, ferrite and bainitic ferrite appear as a graystructure (base steel structure), while retained austenite andmartensite as a white structure.

Identification of ferrite and bainitic ferrite is made by EBSD (ElectronBackscatter Diffraction) measurement. A crystal grain (phase) thatincludes a sub-boundary with a grain boundary angle of smaller than 15°is identified as bainitic ferrite, for which the area ratio iscalculated and the result is used as the area ratio of bainitic ferrite.The area ratio of ferrite is calculated by subtracting the area ratio ofbainitic ferrite from the area ratio of the above-described graystructure.

Area Ratio of Martensite: 3% or More and 20% or Less

According to the disclosure, to ensure the strength of the steel sheet,the area ratio of martensite needs to be 3% or more. On the other hand,to ensure the steel sheet has good ductility, the area ratio ofmartensite needs to be 20% or less. For obtaining better ductility andstretch flangeability, the area ratio of martensite is preferably 15% orless.

Note that “the area ratio of martensite” is calculated with thefollowing method. Firstly, polish an L-cross section of the steel sheet,etch the L-cross section with 3 vol. % nital, and observe ten locationsat 2000 times magnification under an SEM at a position of sheetthickness×¼ (a position at a depth of one-fourth of the sheet thicknessfrom the steel sheet surface). Then, using the structure micrographsimaged with the SEM, calculate the total area ratio of martensite andretained austenite, both appearing white, for the ten locations withImage-Pro described above. Then, average the results, subtract the arearatio of retained austenite from the average, and use the result as “thearea ratio of martensite.” In the structure micrographs, martensite andretained austenite appear as a white structure. As used herein, as thearea ratio of retained austenite, the volume fraction of retainedaustenite described below is used.

Volume Fraction of Retained Austenite: 10% or More

According to the disclosure, to ensure good ductility and balancestrength and ductility, the volume fraction of retained austenite needsto be 10% or more. For obtaining better ductility and achieving a betterbalance between strength and ductility, it is preferred that the volumefraction of retained austenite is 12% or more.

The volume fraction of retained austenite is calculated by determiningthe x-ray diffraction intensity of a plane of sheet thickness×¼, whichis exposed by polishing the steel sheet surface to a depth of one-fourthof the sheet thickness. Using an incident x-ray beam of MoKα, theintensity ratio of the peak integrated intensity of the {111}, {200},{220}, and {311} planes of retained austenite to the peak integratedintensity of the {110}, {200}, and {211} planes of ferrite is calculatedfor all of the twelve combinations, the results are averaged, and theaverage is used as the volume fraction of retained austenite.

Mean Grain Size of Retained Austenite: 2 μm or Less

Refinement of retained austenite grains contributes to improving theductility of the steel sheet and the stability as a material.Accordingly, to ensure good ductility of the steel sheet and stabilityas a material, the mean grain size of retained austenite needs to be 2μm or less. For obtaining better ductility and stability as a material,the mean grain size of retained austenite is preferably 1.5 μm or less.

As used herein, “the mean grain size of retained austenite” iscalculated with the following method. First, observe twenty locations at15000 times magnification under a TEM (transmission electronmicroscope), and image structure micrographs. Then, calculate equivalentcircular diameters from the areas of retained austenite grainsidentified with Image-Pro as mentioned above in the structuremicrographs for the twenty locations, average the results, and use theaverage as “the mean grain size of retained austenite.” For theabove-described observation, the steel sheet was cut from both front andback surfaces up to 0.3 mm thick, so that the central portion in thesheet thickness direction of the steel sheet is located at a position ofsheet thickness×¼. Then, electropolishing was performed on the front andback surfaces to form a hole, and a portion reduced in sheet thicknessaround the hole was observed under the TEM in the sheet surfacedirection.

The Mean Mn Content in Retained Austenite (in Mass %) is at Least 1.2Times the Mn Content in the Steel Sheet (in Mass %).

This is one of the very important controllable factors for thedisclosure. The reason is as follows. When the mean Mn content inretained austenite (in mass %) is at least 1.2 times the Mn content inthe steel sheet (in mass %), and when a single phase of martensite, asingle phase of bainite, or a mixed phase of martensite and bainite isdominantly present in the microstructure prior to second annealing,carbides with Mn concentrated therein precipitate in the first placewhen raising the temperature during second annealing. Then, the carbidesact as nuclei for austenite through reverse transformation, andeventually fine retained austenite is uniformly distributed in themicrostructure, improving the stability of the steel sheet as amaterial. The mean Mn content in retained austenite can be measured byanalysis with FE-EPMA (Field Emission-Electron Probe Micro Analyzer).

No upper limit is particularly placed on the mean Mn content in retainedaustenite (in mass %) as long as the mean Mn content in retainedaustenite is at least 1.2 times the Mn content in the steel sheet (inmass %). However, it is preferred that the mean Mn content in retainedaustenite is about 2.5 times the Mn content in the steel sheet, in mass%.

The Area Ratio of Retained Austenite Having a Mean C Content (in Mass %)at Least 2.1 Times the C Content in the Steel Sheet (in Mass %) is 60%or More of the Area Ratio of the Entire Retained Austenite.

To ensure good ductility by guaranteeing the formation of a desiredvolume fraction of stable retained austenite, the area ratio of retainedaustenite having a mean C content (in mass %) at least 2.1 times the Ccontent in the steel sheet (in mass %) needs to be 60% or more of thearea ratio of the entire retained austenite.

This requirement is not satisfied after performing annealing treatmentonly once, but is satisfied after performing annealing treatment twice.

No upper limit is particularly placed on the area ratio of retainedaustenite having a mean C content (in mass %) at least 2.1 times the Ccontent in the steel sheet (in mass %), yet a preferred upper limit isabout 95%.

According to the disclosure, bainite transformation occurs during thelater half of cooling and austempering treatment after the secondannealing treatment. As a result of such transformation, bainiticferrite is formed in the minor axis direction of acicular austenite todivide the austenite, and fine retained austenite having a mean grainsize of 2 μm or less is formed. The retained austenite formed by thisprocess often has a mean C content (in mass %) that is at least 2.1times the C content in the steel sheet (in mass %), which may ensurevery good ductility.

As used herein, as the area ratio of retained austenite, theabove-described volume fraction of retained austenite is used. In thiscase, the mean Mn content (in mass %) of each phase is calculated byanalysis with FE-EPMA (Field Emission-Electron Probe Micro Analyzer).

In addition to ferrite, bainitic ferrite, martensite, and retainedaustenite, the microstructure according to the disclosure may includecarbides such as tempered martensite, pearlite, cementite, and the like,or other phases well known as steel sheet microstructure constituents.Any of the other phases, such as tempered martensite, may be included aslong as the area ratio is 10% or less, without detracting from theeffect of the disclosure.

The following provides a description of the production method accordingto the disclosure.

To produce the high-strength steel sheet disclosed herein, a steel slabhaving the above-described predetermined chemical composition is heatedto 1100° C. or higher and 1300° C. or lower, and hot rolled with afinisher delivery temperature of 800° C. or higher and 1000° C. or lowerto obtain a steel sheet. Then, the steel sheet is coiled at a meancoiling temperature of 450° C. or higher and 700° C. or lower, subjectedto pickling treatment, and, optionally, retained at a temperature of450° C. or higher and Ac₁ transformation temperature or lower for 900 sor more and 36000 s or less. Then, the steel sheet is cold rolled at arolling reduction of 30% or more, and subjected to first annealingtreatment whereby the steel sheet is heated to a temperature of 820° C.or higher and 950° C. or lower.

Then, the steel sheet is cooled to a first cooling stop temperature ator below Ms under the condition of a mean cooling rate to 500° C. of 15°C./s or higher. Subsequently, the steel sheet is subjected to secondannealing treatment whereby the steel sheet is reheated to a temperatureof 740° C. or higher and 840° C. or lower. Then, the steel sheet iscooled to a temperature in a second cooling stop temperature range of300° C. to 550° C. at a mean cooling rate of 10° C./s or higher and 50°C./s or lower, and retained at the temperature in the second coolingstop temperature range for 10 s or more.

Furthermore, as described below, after being retained at the secondcooling stop temperature range, the steel sheet may be subjected tothird annealing treatment whereby the steel sheet is heated to atemperature of 100° C. or higher and 300° C. or lower.

The high-strength galvanized steel sheet disclosed herein may beproduced by performing well-known and widely-used galvanizing treatmenton the above-mentioned high tensile strength steel sheet.

Steel Slab Heating Temperature: 1100° C. or Higher and 1300° C. or Lower

Precipitates that are present at the time of heating of a steel slabwill remain as coarse precipitates in the resulting steel sheet, makingno contribution to strength. Thus, remelting of any Ti- and Nb-basedprecipitates precipitated during casting is required.

In this respect, if a steel slab is heated at a temperature below 1100°C., it is difficult to cause sufficient melting of carbides, leading toproblems such as an increased risk of trouble during hot rollingresulting from increased rolling load. In addition, for obtaining asmooth steel sheet surface, it is necessary to scale-off defects on thesurface layer of the slab, such as blow hole generation, segregation,and the like, and to reduce cracks and irregularities on the steel sheetsurface. Therefore, according to the disclosure, the steel slab heatingtemperature needs to be 1100° C. or higher. If the steel slab heatingtemperature exceeds 1300° C., however, scale loss increases as oxidationprogresses. Accordingly, the steel slab heating temperature needs to be1300° C. or lower. As such, the slab heating temperature is 1100° C. orhigher and 1300° C. or lower, and preferably 1150° C. or higher and1250° C. or lower.

A steel slab is preferably made with continuous casting to prevent macrosegregation, yet may be produced with other methods such as ingotcasting or thin slab casting. The steel slab thus produced may be cooledto room temperature and then heated again according to the conventionalmethod. Alternatively, there can be employed without problems what iscalled “energy-saving” processes, such as hot direct rolling or directrolling in which either a warm steel slab without being fully cooled toroom temperature is charged into a heating furnace, or a steel slabundergoes heat retaining for a short period and immediately hot rolled.Further, a steel slab is subjected to rough rolling under normalconditions and formed into a sheet bar. When the heating temperature islow, the sheet bar is preferably heated using a bar heater or the likeprior to finish rolling from the viewpoint of preventing troubles duringhot rolling.

Finisher Delivery Temperature in Hot Rolling: 800° C. or Higher and1000° C. or Lower

The heated steel slab is hot rolled through rough rolling and finishrolling to form a hot-rolled steel sheet. At this point, when thefinisher delivery temperature exceeds 1000° C., the amount of oxides(scales) generated suddenly increases and the interface between thesteel substrate and oxides becomes rough, which tends to impair thesurface quality after pickling and cold rolling. In addition, anyhot-rolling scales remaining after pickling adversely affect ductilityand stretch flangeability. Moreover, a grain size is excessivelycoarsened, causing surface deterioration in a pressed part duringworking.

On the other hand, if the finisher delivery temperature is below 800°C., rolling load and burden increase, rolling is performed more often ina state in which recrystallization of austenite does not occur, anabnormal texture develops, and the final product has a significantplanar anisotropy. As a result, the material properties not only becomeless uniform, but the ductility of the steel sheet itself alsodeteriorates.

Therefore, the finisher delivery temperature in hot rolling needs to bein a range of 800° C. to 1000° C., and preferably in a range of 820° C.to 950° C.

Mean Coiling Temperature after Hot Rolling: 450° C. or Higher and 700°C. or Lower

When the mean coiling temperature at which the steel sheet is coiledafter the hot rolling is above 700° C., the grain size of ferrite in themicrostructure of the hot-rolled sheet increases, making it difficult toensure a desired strength of the final-annealed sheet. On the otherhand, when the mean coiling temperature after the hot rolling is below450° C., there is an increase in the strength of the hot-rolled sheetand in the rolling load in cold rolling, degrading productivity.

Therefore, the mean coiling temperature after the hot rolling needs tobe 450° C. or higher and 700° C. or lower, and preferably 450° C. orhigher and 650° C. or lower.

Finish rolling may be performed continuously by joining rough-rolledsheets during the hot rolling. Rough-rolled sheets may be coiled on atemporary basis. At least part of finish rolling may be conducted aslubrication rolling to reduce rolling load in the hot rolling.Conducting lubrication rolling in such a manner is effective from theperspective of making the shape and material properties of the steelsheet uniform. In lubrication rolling, the coefficient of friction ispreferably in a range of 0.10 to 0.25.

The hot-rolled steel sheet thus produced is subjected to pickling.Pickling enables removal of oxides from the steel sheet surface, and isthus important to ensure that the high-strength steel sheet as the finalproduct has good chemical convertibility and a sufficient quality ofcoating. Pickling may be performed in one or more batches.

Heat Treatment Temperature and Holding Time for the Hot-Rolled Sheetafter the Pickling Treatment: Retained at 450° C. or Higher and Ac₁Transformation Temperature or Lower for 900 s or More and 36000 s orLess

When the heat treatment temperature is below 450° C., or when the heattreatment holding time is shorter than 900 s, tempering after the hotrolling of the steel sheet is insufficient, causing a mixed phase offerrite, bainite, and martensite in the microstructure of the steelsheet, and making the microstructure less uniform. Additionally, withsuch microstructure of the hot-rolled sheet, uniform refinement of thesteel sheet microstructure becomes insufficient. This results in anincrease in the proportion of coarse martensite in the microstructure ofthe final-annealed sheet, and thus increases the non-uniformity of themicrostructure, which tends to degrade the final-annealed sheet in termsof hole expansion formability (stretch flangeability) and stability as amaterial.

On the other hand, a heat treatment holding time longer than 36000 s mayadversely affect productivity. In addition, a heat treatment temperatureabove Ac₁ transformation temperature provides a non-uniform, hardened,and coarse dual-phase microstructure of ferrite and either martensite orpearlite, increasing the non-uniformity of the microstructure of thesteel sheet before subjection to cold rolling. This results in anincrease in the proportion of coarse martensite in the final-annealedsheet, which may also degrade the final-annealed sheet in terms of holeexpansion formability (stretch flangeability) and stability as amaterial.

Therefore, for the hot-rolled sheet after subjection to the picklingtreatment, the heat treatment temperature needs to be 450° C. or higherand Ac₁ transformation temperature or lower, and the holding time needsto be 900 s or more and 36000 s or less.

Rolling Reduction During Cold Rolling: 30% or More

When the rolling reduction is below 30%, the number of grain boundariesthat act as nuclei for reverse transformation to austenite and the totalnumber of dislocations per unit area decrease during the subsequentannealing, making it difficult to obtain the above-described resultingmicrostructure. In addition, if the microstructure becomes non-uniform,the ductility of the steel sheet decreases.

Therefore, the rolling reduction during cold rolling needs to be 30% ormore, and is preferably 40% or more. The effect of the disclosure can beobtained without limiting the number of rolling passes or the rollingreduction for each pass. No upper limit is particularly placed on therolling reduction, yet a practical upper limit is about 80% inindustrial terms.

First Annealing Treatment Temperature: 820° C. or Higher 950° C. orLower

If the first annealing temperature range is below 820° C., then the heattreatment is performed at a ferrite-austenite dual phase region, withthe result that a large amount of ferrite (polygonal ferrite) producedat the ferrite-austenite dual phase region will be included in theresulting microstructure. As a result, a desired amount of fine retainedaustenite cannot be produced, making it difficult to balance goodstrength and ductility. On the other hand, when the first annealingtemperature exceeds 950° C., austenite grains are coarsened during theannealing and fine retained austenite cannot be produced eventually,again, making it difficult to balance good strength and ductility. As aresult, productivity decreases.

Without limitation, the holding time during the first annealingtreatment is preferably 10 s or more and 1000 s or less.

Mean Cooling Rate to 500° C. after the First Annealing Treatment: 15°C./s or Higher

This is one of the very important controllable factors for thedisclosure. When the mean cooling rate is below 15° C./s, ferrite andpearlite are produced during the cooling, preventing a low temperaturetransformation phase (bainite or martensite) from being dominantlypresent in the microstructure of the steel sheet before subjection tosecond annealing. As a result, a desired amount of fine retainedaustenite cannot be produced eventually, making it difficult to balancegood strength and ductility. This also reduces the stability of thesteel sheet as a material. No upper limit is particularly placed on themean cooling rate, yet in industrial terms, the mean cooling rate ispractically up to about 60° C./s.

Cooling to a First Cooling Stop Temperature at or Below Ms

In the first annealing treatment, the steel sheet is ultimately cooledto a first cooling stop temperature at or below Ms. The reason is asfollows. With this setup, a single phase of martensite, a single phaseof bainite, or a mixed phase of martensite and bainite is caused to bedominantly present in the microstructure of the steel sheet beforesubjection to second annealing treatment. As a result, during thecooling and retaining process after second annealing, non-polygonalferrite and bainitic ferrite are produced in large amounts withdistorted grain boundaries produced at 600° C. or lower. Consequently,it becomes possible to obtain proper amounts of fine retained austenite,and yield good ductility.

Second Annealing Treatment Temperature: 740° C. or Higher and 840° C. orLower

A second annealing temperature below 740° C. cannot ensure formation ofa sufficient volume fraction of austenite during the annealing, andeventually formation of a desired area ratio of martensite and of adesired volume fraction of retained austenite. Accordingly, it becomesdifficult to ensure strength and to balance good strength and ductility.On the other hand, a second annealing temperature above 840° C. iswithin a temperature range of austenite single phase, and a desiredamount of fine retained austenite cannot be produced in the end. As aresult, this makes it difficult again to ensure good ductility and tobalance strength and ductility. Moreover, unlike the case where heattreatment is performed at a ferrite-austenite dual phase region,distribution of Mn resulting from diffusion hardly occurs. As a result,the mean Mn content in retained austenite (mass %) does not increase toat least 1.2 times the Mn content in the steel sheet (in mass %), makingit difficult to obtain a desired volume fraction of stable retainedaustenite. Without limitation, the holding time during the secondannealing treatment is preferably 10 s or more and 1000 s or less.

Mean Cooling Rate to a Temperature in a Second Cooling Stop TemperatureRange of 300° C. to 550° C.: 10° C./s or Higher and 50° C./s or Lower

When the mean cooling rate to a temperature in a second cooling stoptemperature range of 300° C. to 550° C. after the second annealingtreatment is lower than 10° C./s, a large amount of ferrite forms duringcooling, making it difficult to ensure the formation of bainitic ferriteand martensite. Consequently, it becomes difficult to guarantee thestrength of the steel sheet. On the other hand, when the mean coolingrate is higher than 50° C./s, excessive martensite is produced,degrading the ductility and stretch flangeability of the steel sheet. Inthis case, the cooling is preferably performed by gas cooling; however,furnace cooling, mist cooling, roll cooling, water cooling, and the likecan also be employed in combination.

Holding Time at the Second Cooling Stop Temperature Range (300° C. to550° C.): 10 s or More

If the holding time at the second cooling stop temperature range (300°C. to 550° C.) is shorter than 10 s, there is insufficient time for theconcentration of C (carbon) into austenite to progress, making itdifficult to ensure a desired volume fraction of retained austenite inthe end. Moreover, it becomes difficult to satisfy the condition thatthe area ratio of retained austenite having a mean C content (in mass %)at least 2.1 times the C content in the steel sheet (in mass %) is 60%or more of the area ratio of the entire retained austenite. However, aholding time longer than 600 s does not increase the volume fraction ofretained austenite and ductility does not improve significantly, wherethe effect reaches a plateau. Thus, without limitation, the holding timeis preferably 600 s or less.

Therefore, the holding time at the second cooling stop temperature rangeis 10 s or more, and preferably 600 s or less. Cooling after the holdingis not particularly limited, and any method may be used to implementcooling to a desired temperature. The desired temperature is preferablyaround room temperature.

Third Annealing Treatment Temperature: 100° C. or Higher and 300° C. orLower

When the third annealing treatment is performed at a temperature below100° C., tempering softening of martensite is insufficient, which mayresult in difficulty in ensuring better hole expansion formability(stretch flangeability). On the other hand, if the third annealingtreatment is performed at a temperature above 300° C., decomposition ofretained austenite is caused, which may result in difficulty inguaranteeing a desired volume fraction of retained austenite in the end.Therefore, the third annealing treatment temperature is preferably 100°C. or higher and 300° C. or lower. Without limitation, the holding timeduring the third annealing treatment is preferably 10 s or more and36000 s or less.

Galvanizing Treatment

When hot-dip galvanizing treatment is performed, the steel sheetsubjected to the above-described annealing treatment is immersed in agalvanizing bath at 440° C. or higher and 500° C. or lower for hot-dipgalvanizing, after which coating weight adjustment is performed usinggas wiping or the like. For hot-dip galvanizing, a galvanizing bath withan Al content of 0.10 mass % or more and 0.22 mass % or less ispreferably used. When a galvanized layer is subjected to alloyingtreatment, the alloying treatment is performed in a temperature range of470° C. to 600° C. after the hot-dip galvanizing treatment. If thealloying treatment is performed at a temperature above 600° C.,untransformed austenite transforms to pearlite, where the presence of adesired volume fraction of retained austenite cannot be ensured andductility may degrade. Therefore, when a galvanized layer is subjectedto alloying treatment, the alloying treatment is preferably performed ina temperature range of 470° C. to 600° C. Electrogalvanized plating mayalso be performed.

When skin pass rolling is performed after the heat treatment, the skinpass rolling is preferably performed with a rolling reduction of 0.1% ormore and 1.0% or less. A rolling reduction below 0.1% provides only asmall effect and complicates control, and hence 0.1% is the lower limitof the favorable range. On the other hand, a rolling reduction above1.0% significantly degrades productivity, and thus 1.0% is the upperlimit of the favorable range.

The skin pass rolling may be performed on-line or off-line. Skin passmay be performed in one or more batches with a target rolling reduction.No particular limitations are placed on other manufacturing conditions,yet from the perspective of productivity, the aforementioned series ofprocesses such as annealing, hot-dip galvanizing, and alloying treatmenton a galvanized layer are preferably carried out on a CGL (ContinuousGalvanizing Line) as the hot-dip galvanizing line. After the hot-dipgalvanizing, wiping may be performed for adjusting the coating amounts.Conditions other than the above, such as coating conditions, may bedetermined in accordance with conventional hot-dip galvanizing methods.

EXAMPLES

Steels having the chemical compositions presented in Table 1, each withthe balance consisting of Fe and incidental impurities, were prepared bysteelmaking in a converter and formed into slabs by continuous casting.The steel slabs thus obtained were heated under the conditions presentedin Table 2, and subjected to hot rolling to obtain steel sheets. Thesteel sheets were then subjected to pickling treatment. Then, for SteelNos. 1-22, 24, 25, 28, 30, 31, 33, 35-40, 42, and 44-56 presented inTable 2, heat treatment was performed on the hot-rolled sheets under theconditions presented in Table 2. Out of these, for Steel Nos. 31, 33,35-40, 42, and 44, the steel sheets were subjected to pickling treatmentafter subjection to the heat treatment.

Then, cold rolling was performed on the steel sheets under theconditions presented in Table 2. Subsequently, annealing treatment wasconducted on the steel sheets two or three times under the conditions inTable 2 to produce high-strength cold-rolled steel sheets (CR).

Moreover, some of the high-strength cold-rolled steel sheets (CR) weresubjected to galvanizing treatment to obtain hot-dip galvanized steelsheets (GI), galvannealed steel sheets (GA), electrogalvanized steelsheets (EG), and so on. Used as hot-dip galvanizing baths were a zincbath containing 0.19 mass % of Al for GI and a zinc bath containing 0.14mass % of Al for GA, in each case the bath temperature was 465° C. Thecoating weight per side was 45 g/m² (in the case of both-sided coating),and the Fe concentration in the coated layer of each hot-dipgalvannealed steel sheet (GA) was 9 mass % or more and 12 mass % orless.

The Ac₁ transformation temperature (° C.) presented in Table 1 wascalculated by:

Ac ₁ transformation temperature (° C.)=751−16×(% C)+11×(% Si)−28×(%Mn)−5.5×(% Cu)+13×(% Cr)

Where (% X) represents content (in mass %) of an element X in steel.

Ms (° C.) presented in Table 3 was calculated by:

Ms (° C.)=550−361×(% C)×0.01×[fraction of A (%)immediately afterannealing in second annealing treatment]−69×[Mn content in retainedaustenite (%)]−20×(% Cr)−10×(% Cu)+30×(% Al)

Where (% X) represents content (in mass %) of an element X in steel.

Here, “fraction of A (%) immediately after annealing in second annealingtreatment” is defined as the area ratio of martensite in themicrostructure of the steel sheet subjected to water quenching (meancooling rate to room temperature: 800° C./s or higher) immediately aftersubjection to annealing in second annealing treatment (temperaturerange: 740° C. to 840° C.). The area ratio of martensite can becalculated with the above-described method.

In the above expression, “Mn content in retained austenite (%)” is themean Mn content in retained austenite (mass %) of the resultinghigh-strength steel sheet.

TABLE 1 Steel Chemical composition (mass %) ID C Si Mn P S N Al Ti Nb CrA 0.091 1.62 2.38 0.021 0.0020 0.0032 — — — — B 0.154 1.24 2.11 0.0190.0019 0.0030 — — — — C 0.208 1.28 2.04 0.014 0.0018 0.0032 — — — — D0.232 0.71 2.34 0.025 0.0022 0.0030 — — — — E 0.224 0.98 2.01 0.0290.0016 0.0032 — — — — F 0.218 1.45 1.94 0.016 0.0024 0.0033 — — — — G0.228 1.55 1.69 0.018 0.0019 0.0034 — — — — H 0.200 1.48 2.01 0.0220.0021 0.0030 — — — — I 0.182 1.39 2.72 0.028 0.0019 0.0029 — — — — J0.058 1.51 2.89 0.027 0.0018 0.0028 — — — — K 0.232 0.31 2.78 0.0230.0021 0.0030 — — — — L 0.213 1.43 1.22 0.028 0.0028 0.0028 — — — — M0.202 1.34 2.22 0.018 0.0024 0.0034 0.430 — — — N 0.198 1.22 1.94 0.0310.0022 0.0031 — 0.039 — — O 0.188 1.24 1.87 0.016 0.0026 0.0032 — —0.042 — P 0.234 1.48 1.96 0.028 0.0018 0.0030 — — — 0.21 Q 0.203 1.462.21 0.015 0.0024 0.0029 — — — — R 0.221 1.49 2.18 0.024 0.0019 0.0033 —— — — S 0.187 1.56 1.98 0.019 0.0028 0.0034 — — — — T 0.189 1.45 2.030.024 0.0018 0.0029 — — — — U 0.199 1.32 2.09 0.025 0.0017 0.0044 — —0.032 — V 0.202 1.38 2.12 0.018 0.0026 0.0036 — — 0.024 — W 0.211 1.461.97 0.028 0.0025 0.0042 — — 0.041 — X 0.213 1.24 1.93 0.019 0.00220.0044 — — — — Y 0.197 1.44 2.21 0.024 0.0019 0.0036 — — — — Z 0.1981.63 2.09 0.020 0.0017 0.0032 — — — — AA 0.083 1.29 1.72 0.014 0.00420.0042 — — — — AB 0.086 1.51 2.96 0.019 0.0022 0.0043 — — — — AC 0.0860.88 1.62 0.021 0.0053 0.0045 — — — — AD 0.092 0.93 2.92 0.024 0.00210.0039 — — — — AE 0.089 2.39 2.87 0.025 0.0056 0.0046 — — — — AF 0.3101.25 1.69 0.015 0.0042 0.0051 — — — — AG 0.293 1.32 2.41 0.018 0.00190.0041 — — — — AH 0.289 1.41 2.89 0.022 0.0022 0.0031 — — — — AI 0.1201.49 2.39 0.021 0.0030 0.0038 — 0.078 — — AJ 0.176 1.41 2.67 0.0060.0029 0.0040 — — — — AK 0.189 1.56 2.62 0.018 0.0009 0.0032 — — — — AL0.223 1.39 2.31 0.007 0.0008 0.0038 — — — — Ac₁ transformation Steeltemperature ID Cu Sb Sn Ta Ca Mg REM (° C.) Remarks A — — — — — — — 701Disclosed Steel B — — — — — — — 703 Disclosed Steel C — — — — — — — 705Disclosed Steel D — — — — — — — 690 Disclosed Steel E — — — — — — — 702Disclosed Steel F — — — — — — — 709 Disclosed Steel G — — — — — — — 717Disclosed Steel H — — — — — — — 708 Disclosed Steel I — — — — — — — 687Disclosed Steel J — — — — — — — 686 Comparative Steel K — — — — — — —673 Comparative Steel L — — — — — — — 729 Comparative Steel M — — — — —— — 700 Disclosed Steel N — — — — — — — 707 Disclosed Steel O — — — — —— — 709 Disclosed Steel P — — — — — — — 711 Disclosed Steel Q 0.24 — — —— — — 701 Disclosed Steel R — 0.0042 — — — — — 703 Disclosed Steel S — —0.0048 — — — — 710 Disclosed Steel T — — — 0.0037 — — — 707 DisclosedSteel U — 0.0062 — — — — — 704 Disclosed Steel V — — 0.0068 — — — — 704Disclosed Steel W — — — 0.0057 — — — 709 Disclosed Steel X — — — —0.0029 — — 707 Disclosed Steel Y — — — — — 0.0018 — 702 Disclosed SteelZ — — — — — — 0.0024 707 Disclosed Steel AA — — — — — — — 716 DisclosedSteel AB — — — — — — — 683 Disclosed Steel AC — — — — — — — 714Disclosed Steel AD — — — — — — — 678 Disclosed Steel AE — — — — — — —696 Disclosed Steel AF — — — — — — — 712 Disclosed Steel AG — — — — — —— 693 Disclosed Steel AH — — — — — — — 681 Disclosed Steel AI — — — — —— — 699 Disclosed Steel AJ — — — — — — — 689 Disclosed Steel AK — — — —— — — 692 Disclosed Steel AL — — — — — — — 698 Disclosed SteelUnderlined if outside of the disclosed range.

TABLE 2 Second annealing treatment Holding time Heat First annealingtreatment at temp. Third treatment on Rolling Mean range annealingHot-rolling treatment hot rolled sheet reduction cooling of treatmentSlab Finisher Mean Heat Heat in An- rate Cooling An- Mean Cooling 300°C. An- heating delivery coiling treatment treatment cold nealing Holdingto stop nealing Holding cooling stop to nealing Holding Steel temp.temp. temp. temp. time rolling temp. time 500° C. temp. temp. time ratetemp. 550° C. temp. time No. ID (° C.) (° C.) (° C.) (° C.) (s) (%) (°C.) (s) (° C./s) (° C.) (° C.) (s) (° C./s) (° C.) (s) (° C.) (s) Type*Remark 1 A 1230 900 550 550 18000 62.5 880 100 20 250 780 200 15 400 180200 20000 CR Example 2 B 1250 890 580 500 22000 56.3 860 90 18 30 800100 18 420 150 — — GI Example 3 C 1220 910 500 500 22000 50.0 900 180 22150 800 150 20 450 200 — — GI Example 4 C 900 900 600 550 23000 52.6 890200 30 80 790 200 13 380 140 — — CR Comparative Example 5 C 1400 900 560550 16000 64.7 880 300 25 50 770 80 14 500 120 — — CR ComparativeExample 6 C 1230 650 580 540 18000 58.8 870 250 20 200 810 200 15 420200 — — CR Comparative Example 7 C 1220 1150 530 530 25000 56.3 900 10018 150 800 250 15 400 300 — — CR Comparative Example 8 C 1240 900 300520 18000 57.1 900 150 22 220 800 200 16 450 200 — — GI ComparativeExample 9 C 1250 890 800 550 24000 56.5 860 200 29 90 790 100 20 460 220220 6000 CR Comparative Example 10 C 1230 900 560 540 18000 21.7 870 6030 250 820 180 15 500 250 — — CR Comparative Example 11 C 1220 910 550520 15000 56.3 750 120 27 80 790 160 13 420 150 — — EG ComparativeExample 12 C 1220 870 510 500 18000 60.0 1000 300 28 30 760 200 14 400200 — — CR Comparative Example 13 C 1250 860 490 490 20000 57.1 880 2503 250 770 150 15 380 300 180 18000 CR Comparative Example 14 C 1250 900600 500 22000 53.8 870 200 18 210 660 400 15 410 250 — — CR ComparativeExample 15 C 1240 890 570 520 16000 50.0 900 500 20 160 900 500 16 430200 — — CR Comparative Example 16 C 1220 900 570 580 23000 56.3 860 40025 200 800 300 70 400 180 — — EG Comparative Example 17 C 1250 900 560560 25000 53.8 870 300 30 80 810 150 30 250 8 — — GI Comparative Example18 C 1220 870 550 560 18000 56.3 880 80 24 40 800 200 13 650 — — — CRComparative Example 19 C 1220 890 510 550 24000 52.9 890 150 25 120 790250 16 420 8 — — GA Comparative Example 20 C 1220 900 490 550 20000 57.1880 250 26 50 790 100 18 400 900 — — GI Example 21 C 1240 910 600 60018000 57.1 900 300 20 250 770 250 18 420 300 200 25000 CR Example 22 D1220 890 620 550 23000 52.9 880 200 24 200 800 150 22 480 250 — — CRExample 23 E 1250 900 540 — — 47.1 890 150 26 230 790 250 22 420 260 20020000 CR Example 24 F 1240 910 660 530 18000 50.0 880 100 27 50 780 10020 400 270 — — GA Example 25 G 1230 870 590 520 24000 47.8 870 200 30 70790 80 18 480 190 — — GI Example 26 H 1210 860 590 — — 47.8 900 250 21100 800 300 20 500 160 — — EG Example 27 I 1220 870 590 — — 56.3 900 20021 30 820 280 17 380 150 — — CR Example 28 J 1220 860 590 570 18000 64.7880 100 18 270 800 100 20 400 190 220 8000 CR Comparative Example 29 K1220 900 590 — — 64.7 870 150 19 200 790 150 15 400 500 — — EGComparative Example 30 L 1230 890 590 560 20000 56.3 880 200 20 250 800200 14 420 200 — — CR Comparative Example 31 M 1250 910 590 560 1800064.7 870 250 25 100 810 250 16 450 450 — — GI Example 32 N 1260 900 580— — 50.0 900 200 26 250 820 200 14 380 180 180 4000 CR Example 33 O 1200880 500 550 15000 46.2 880 180 30 120 790 190 25 500 150 200 15000 CRExample 34 P 1250 870 600 — — 52.9 860 160 24 260 760 200 24 420 550 — —CR Example 35 Q 1240 890 560 500 20000 47.1 900 200 21 80 780 300 15 400300 — — EG Example 36 R 1230 900 580 550 22000 52.9 890 180 20 190 800100 26 410 250 — — GA Example 37 S 1230 910 560 500 20000 61.1 880 20018 300 820 150 15 500 190 — — GI Example 38 T 1210 880 550 550 1500058.8 880 100 19 250 810 190 15 400 300 — — EG Example 39 U 1220 900 510520 18000 57.1 900 120 18 60 800 250 14 420 540 — — GI Example 40 V 1220890 490 480 20000 64.7 900 80 18 100 800 200 13 480 250 220 20000 CRExample 41 W 1220 900 600 — — 58.8 870 300 20 35 800 250 14 500 350 — —EG Example 42 X 1230 910 520 520 16000 57.1 880 400 21 80 790 180 20 430200 — — GA Example 43 Y 1240 900 530 — — 58.8 890 140 26 120 760 140 18400 200 — — GI Example 44 Z 1210 860 540 580 16000 50.0 900 200 27 230800 90 18 410 180 180 20000 CR Example 45 AA 1230 920 580 620 22000 64.3910 250 22 40 810 200 16 380 210 220 8000 CR Example 46 AB 1210 890 650580 16000 61.1 880 180 30 80 790 160 22 480 150 200 15000 GA Example 47AC 1260 870 600 610 24000 50.0 860 150 27 200 820 250 24 420 450 — — CRExample 48 AD 1240 890 560 500 28000 57.1 880 220 23 50 770 300 18 420300 — — CR Example 49 AE 1230 870 580 590 20000 50.0 890 300 20 100 750120 23 400 300 — — GA Example 50 AF 1210 830 660 630 30000 57.1 930 20017 30 830 140 17 480 180 250 17000 GI Example 51 AG 1200 880 550 55012000 39.5 880 100 19 30 810 190 13 370 280 — — EG Example 52 AH 1190900 510 640 18000 42.9 850 140 22 70 800 300 35 500 500 — — CR Example53 AI 1220 840 490 490 24000 35.7 900 80 18 100 820 180 40 400 280 16023000 CR Example 54 AJ 1230 910 600 620 29000 57.1 910 400 25 25 800 24025 420 140 — — GI Example 55 AK 1240 850 520 520 11000 58.8 880 200 2680 770 200 23 390 180 210 20000 GA Example 56 AL 1250 900 530 600 1700050.0 870 140 20 100 790 150 20 490 260 — — CR Example Underlined ifoutside of the disclosed range. *CR: cold-rolled steel (uncoated), GI:hot-dip galvanized steel sheets (alloying treatment not performed ongalvanized layers), GA: galvannealed steel sheets, EG: electrogalvanizedsteel sheets

TABLE 3 Microstructure Mn content Surface Mn in Sheet Sheet charac- Meancontent RA/ C passage passage teristics grain Mn in Mn con- abilityability of cold- Area Area Volume size content steel content tent Sheetduring during rolled ratio of ratio of fraction of in sheet in in RASteel thickness hot cold steel Produc- F + BF M of RA RA RA (mass steel(mass No. ID (mm) rolling rolling sheet tivity (%) (%) (%) (μm) (mass %)%) sheet %) 1 A 1.2 High High Good High 74.8 8.8 15.4 0.7 3.12 2.38 1.310.39 2 B 1.4 High High Good High 72.5 8.9 17.8 0.8 3.04 2.11 1.44 0.64 3C 1.6 High High Good High 70.2 7.9 19.2 0.8 2.89 2.04 1.42 0.69 4 C 1.8Low Low Poor Low 68.6 9.8 16.9 1.3 2.55 2.04 1.25 0.59 5 C 1.2 Low LowPoor Low 67.2 9.5 16.4 2.4 2.58 2.04 1.26 0.61 6 C 1.4 Low Low Poor High64.2 5.9 8.2 0.5 2.48 2.04 1.22 0.54 7 C 1.4 High Low Poor Low 70.7 10.412.2 2.8 2.46 2.4 1.21 0.45 8 C 1.2 High Low Good Low 69.9 12.7 15.0 2.22.54 2.04 1.25 0.51 9 C 1.0 High High Good High 75.6 6.8 4.2 0.4 2.712.04 1.33 0.49 10 C 1.8 High High Good High 72.8 10.2 9.1 2.5 2.59 2.041.27 0.64 11 C 1.4 High High Good High 69.1 20.8 5.8 2.8 2.46 2.04 1.210.51 12 C 1.2 High High Good High 72.4 8.5 13.4 3.2 2.22 2.04 1.09 0.5413 C 1.2 High High Good High 72.4 18.2 6.8 3.0 2.49 2.04 1.22 0.53 14 C1.2 High High Good High 84.4 2.1 3.2 1.5 2.56 2.04 1.25 0.51 15 C 1.4High High Good High 66.9 22.5 5.2 3.1 2.21 2.04 1.08 0.52 16 C 1.4 HighHigh Good High 59.5 28.4 11.1 1.6 2.56 2.04 1.25 0.49 17 C 1.2 High HighGood High 68.2 10.4 3.2 3.1 2.59 2.04 1.27 0.39 18 C 1.4 High High GoodHigh 69.7 23.4 2.8 0.4 2.55 2.04 1.25 0.40 19 C 1.6 High High Good High68.8 21.1 3.9 0.5 2.62 2.04 1.28 0.41 20 C 1.2 High High Good Middle71.4 10.4 16.8 0.7 2.69 2.04 1.32 0.58 21 C 1.2 High High Good High 69.88.1 19.4 0.6 2.94 2.04 1.44 0.72 22 D 1.6 High High Good High 65.8 11.920.9 1.2 3.55 2.34 1.52 0.78 23 E 1.8 High High Good High 72.2 8.9 17.81.0 2.89 2.01 1.44 0.69 24 F 1.4 High High Good High 71.4 9.6 18.2 0.82.78 1.94 1.43 0.72 25 G 1.2 High High Good High 72.9 6.2 20.6 0.6 2.321.69 1.37 0.79 26 H 1.2 High High Good High 71.1 9.2 18.4 0.9 2.82 2.011.40 0.71 27 I 1.4 High High Good High 59.2 14.8 24.8 0.7 3.78 2.72 1.390.58 28 J 1.2 High High Good High 72.4 1.8 2.3 0.3 3.55 2.89 1.23 0.1329 K 1.2 High High Good High 62.7 30.2 3.2 0.5 3.44 2.78 1.24 0.51 30 L1.4 High High Good High 66.8 2.2 4.5 0.6 1.68 1.22 1.38 0.52 31 M 1.2High High Good High 70.1 10.1 19.2 0.9 2.85 2.22 1.28 0.68 32 N 1.4 HighHIgh Good High 71.2 9.4 18.9 0.8 2.77 1.94 1.43 0.69 33 O 1.4 High HighGood High 69.1 10.5 20.2 1.0 2.89 1.87 1.55 0.64 34 P 1.6 High High GoodHigh 72.4 8.3 18.4 0.9 2.92 1.96 1.49 0.76 35 Q 1.8 High High Good High69.2 10.9 18.7 1.0 3.02 2.21 1.37 0.71 36 R 1.6 High High Good High 72.97.9 18.1 0.7 2.89 2.18 1.33 0.75 37 S 1.4 High High Good High 76.4 6.514.6 0.5 27.2 1.98 1.37 0.75 38 T 1.4 High High Good High 74.4 6.6 17.60.6 2.75 2.03 1.35 0.70 39 U 1.2 High High Good High 72.3 8.2 18.9 0.72.81 2.09 1.34 0.65 40 V 1.2 High High Good High 70.1 9.8 20.0 0.6 3.022.12 1.42 0.68 41 W 1.4 High High Good High 67.7 10.4 21.6 0.6 2.68 1.971.36 0.70 42 X 1.2 High High Good High 73.1 7.9 18.4 0.8 2.69 1.93 1.390.72 43 Y 1.4 High High Good High 70.8 8.6 19.5 0.9 3.12 2.21 1.41 0.6944 Z 1.4 High High Good High 72.1 7.5 19.7 0.7 2.88 2.09 1.38 0.67 45 AA1.0 High High Good High 74.8 9.4 13.8 0.8 2.94 1.72 1.71 0.36 46 AB 1.4High High Good High 68.4 14.6 12.3 1.0 4.89 2.96 1.65 0.34 47 AC 1.6High High Good High 70.9 13.5 11.4 1.3 2.77 1.62 1.71 0.32 48 AD 1.2High High Good High 67.1 15.2 13.2 1.1 4.91 2.92 1.68 0.35 49 AE 2.0High High Good High 68.6 12.9 17.1 0.9 4.65 2.87 1.62 0.38 50 AF 1.2High High Good High 67.4 9.6 22.1 0.7 2.72 1.69 1.61 1.12 51 AG 2.3 HighHigh Good High 65.9 10.5 22.4 0.6 3.75 2.41 1.56 1.03 52 AH 1.6 HIghHIgh Good High 62.7 12.8 23.9 0.8 4.45 2.89 1.54 1.09 53 AI 1.8 HighHigh Good High 69.6 10.1 18.7 0.9 4.07 2.39 1.70 0.61 54 AJ 1.2 HighHigh Good High 66.8 11.9 20.4 0.7 4.68 2.67 1.75 0.87 55 AK 1.4 HighHigh Good High 65.9 10.8 22.3 0.7 4.72 2.62 1.80 0.89 56 AL 1.6 HighHigh Good High 62.1 12.4 24.8 0.8 4.12 2.31 1.78 1.13 MicrostructureArea ratio of RA to entire RA, Fraction with C of A C content immedi- Ccontent in RA/C ately content in content after in RA/C in annealingsteel content steel TS × in second sheet in sheet ≧ EL annealing (masssteel 2.1 Balance TS EL (MPa λ ΔTS*1 ΔEL*2 treatment Ms No. %) sheet (%)structure (MPa) (%) %) (%) (MPa) (%) (%) (° C.) Remarks 1 0.091 4.29 80TM + P + θ 798 40.2 32080 55 12 0.9 64.2 314 Example 2 0.154 4.16 82TM + P + θ 904 3.76 33990 43 15 10 66.7 303 Example 3 0.208 3.32 81 TM +P + θ 1004 33.8 33935 39 16 1.2 67.1 300 Example 4 0.208 2.84 48 TM +P + θ 1028 25.9 26625 31 30 2.4 66.7 324 Comparative Example 5 0.2082.93 66 TM + P + θ 1035 25.5 26393 32 48 3.6 35.9 322 ComparativeExample 6 0.208 2.60 68 TM + P + θ 1224 12.4 15178 11 62 4.8 54.1 338Comparative Example 7 0.208 2.16 62 TM + P + θ 1008 18.9 19051 18 36 2.662.6 333 Comparative Example 8 0.208 2.45 44 TM + P + θ 942 27.4 2581140 42 3.2 67.7 324 Comparative Example 9 0.208 2.36 68 TM + P + θ 68134.2 23290 40 28 2.0 51.0 325 Comparative Example 10 0.208 3.08 69 TM +P + θ 1045 15.8 16511 30 32 2.2 59.3 327 Comparative Example 11 0.2082.45 74 TM + P + θ 1192 16.2 19310 20 34 2.4 66.6 330 ComparativeExample 12 0.208 2.60 68 TM + P + θ 1022 18.4 18805 31 30 2.1 61.9 350Comparative Example 13 0.208 2.55 72 TM + P + θ 1279 14.8 18929 28 634.2 65.0 329 Comparative Example 14 0.208 2.45 75 TM + P + θ 682 26.918346 43 28 2.1 45.3 339 Comparative Example 15 0.208 2.50 68 TM + P + θ1087 16.7 18153 30 31 2.0 67.7 347 Comparative Example 16 0.208 2.36 82TM + P + θ 1189 15.8 18786 10 34 2.4 79.5 314 Comparative Example 170.208 1.88 48 TM + P + θ 1089 16.7 18186 38 30 2.1 53.6 331 ComparativeExample 18 0.208 1.92 42 TM + P + θ 1192 15.8 18834 12 32 2.5 66.2 324Comparative Example 19 0.208 1.97 51 TM + P + θ 1198 14.9 17850 11 322.3 65.0 320 Comparative Example 20 0.208 2.79 69 TM + P + θ 1042 29.430635 32 27 1.9 67.2 314 Example 21 0.208 3.46 86 TM + P + θ 1024 32.433178 60 11 1.0 67.5 296 Example 22 0.232 3.36 72 TM + P + θ 1102 29.732729 34 22 1.6 72.8 244 Example 23 0.224 3.08 78 TM + P + θ 998 33.533433 48 17 1.3 66.7 297 Example 24 0.218 3.30 82 TM + P + θ 1039 30.932105 36 16 1.2 67.8 305 Example 25 0.228 3.46 84 TM + P + θ 989 34.634219 44 12 0.9 66.8 335 Example 26 0.200 3.55 74 TM + P + θ 1002 32.932966 41 16 1.1 67.6 307 Example 27 0.182 3.19 70 TM + P + θ 1204 26.431786 29 28 2.0 79.6 237 Example 28 0.058 2.24 64 TM + P + θ 692 26.818546 56 32 2.5 44.1 296 Comparative Example 29 0.232 2.20 68 TM + P + θ1230 11.2 13776 12 68 5.2 73.4 251 Comparative Example 30 0.213 2.44 70TM + P + θ 681 27.6 18796 48 30 2.4 46.7 398 Comparative Example 310.202 3.37 76 TM + P + θ 1051 29.8 31320 34 18 1.3 69.3 316 Example 320.198 3.48 77 TM + P + θ 1042 30.4 31677 41 17 1.4 68.3 310 Example 330.188 3.40 72 TM + P + θ 1065 29.2 31098 42 19 1.5 70.7 303 Example 340.234 3.25 74 TM + P + θ 999 33.8 33766 42 12 1.1 66.7 288 Example 350.203 3.50 77 TM + P + θ 1002 32.5 32565 34 15 1.2 69.6 288 Example 360.221 3.39 74 TM + P + θ 999 33.8 33766 42 12 1.1 66.0 298 Example 370.187 4.01 84 TM + P + θ 822 38.4 31565 50 9 0.8 61.1 321 Example 380.189 3.70 79 TM + P + θ 903 34.5 31154 48 10 0.9 64.2 316 Example 390.199 3.27 78 TM + P + θ 996 33.2 33067 40 13 1.2 67.1 308 Example 400.202 3.37 76 TM + P + θ 1028 32.6 33513 42 16 1.6 69.8 291 Example 410.211 3.32 72 TM + P + θ 1102 29.6 32619 33 20 1.8 72.0 310 Example 420.213 3.38 70 TM + P + θ 997 33.7 33599 41 12 1.2 66.3 313 Example 430.197 3.50 74 TM + P + θ 1034 32.1 33191 36 16 1.5 68.1 286 Example 440.198 3.38 76 TM + P + θ 1021 32.4 33080 45 14 1.4 67.2 303 Example 450.083 4.34 69 TM + P + θ 812 35.6 28907 44 18 1.2 58.2 330 Example 460.086 3.95 71 TM + P + θ 1023 28.2 28849 38 16 1.4 61.9 193 Example 470.086 3.72 67 TM + P + θ 788 34.7 27344 42 28 1.8 59.9 340 Example 480.092 3.80 66 TM + P + θ 992 27.8 27578 36 26 2.0 63.4 190 Example 490.089 4.27 72 TM + P + θ 1184 24.2 28653 28 24 1.6 65.0 208 Example 500.310 3.61 77 TM + P + θ 1063 30.8 32740 32 22 1.4 66.7 288 Example 510.293 3.52 79 TM + P + θ 1129 28.9 32628 34 26 1.8 67.9 219 Example 520.289 3.77 80 TM + P + θ 1229 27.4 33675 25 30 2.2 71.7 168 Example 530.120 508 78 TM + P + θ 986 29.7 29284 35 20 1.7 63.8 242 Example 540.176 4.94 76 TM + P + θ 1145 27.8 31831 31 18 1.3 67.3 184 Example 550.189 4.71 79 TM + P + θ 1132 28.9 32715 32 22 1.5 68.1 178 Example 560.223 5.07 85 TM + P + θ 1086 32.6 35404 36 21 1.4 72.2 208 ExampleUnderlined if outside the disclosed range *1ΔTS upon the secondannealing temperature changing by 40° C. (±20° C.). *2ΔEL upon thesecond annealing temperature changing by 40° C. (±20° C.). F: ferrite,BF: bainitic ferrrite, RA: retained austenite, M: martensite, TN:tempered martensite, P: pearlite, θ: cementite, A: austenite

The obtained steel sheets, such as high-strength cold-rolled steelsheets (CR), hot-dip galvanized steel sheets (GI), galvannealed steelsheets (GA), electrogalvanized steel sheet (EG), and the like, weresubjected to tensile test and hole expansion test.

Tensile test was performed in accordance with JIS Z 2241 (2011) tomeasure TS (tensile strength) and EL (total elongation), using JIS No. 5test pieces that were sampled such that the longitudinal direction ofeach test piece coincides with a direction perpendicular to the rollingdirection of the steel sheet (the C direction). In this case, TS and ELwere determined to be good when EL 34% for TS 780 MPa grade, EL≧27% forTS 980 MPa grade, and EL≧23% for TS 1180 MPa grade, and TS×EL≧27000MPa·%.

Hole expansion test was performed in accordance with JIS Z 2256 (2010).Each of the steel sheets thus obtained was cut to a sample size of 100mm×100 mm, and a hole with a diameter of 10 mm was drilled through eachsample with clearance 12%±1%. Subsequently, each steel sheet was clampedinto a die having an inner diameter of 75 mm with a blank holding forceof 8 tons (7.845 kN). In this state, a conical punch of 60° was pushedinto the hole, and the hole diameter at crack initiation limit wasmeasured. Based on the measured hole diameter, the maximum holeexpansion ratio λ (%) was calculated by the following equation toevaluate hole expansion formability:

maximum hole expansion ratio λ(%)={(D _(f) −D ₀)/D ₀}×100

Where D_(f) is a hole diameter at the time of occurrence of cracking(mm) and D₀ is an initial hole diameter (mm).

In this case, the hole expansion formability was determined to be goodwhen λ≧40% for TS 780 MPa grade, λ≧30% for TS 980 MPa grade, and λ≧20%for TS 1180 MPa grade.

Regarding the stability as a material, for Steel Nos. 1-56, equivalenthigh-strength cold-rolled steel sheets were produced at different secondannealing temperatures±20° C., and TS and EL were measured.

In this case, TS and EL were determined to be good when ΔTS, which isthe amount of variation of TS upon the annealing temperature duringsecond annealing treatment changing by 40° C. (±20° C.), is 36 MPa orless, and ΔEL, which is the amount of variation of EL upon the annealingtemperature changing by 40° C., is 2.4% or less.

The sheet passage ability during hot rolling was determined to be lowwhen the risk of trouble during hot rolling increased with increasingrolling load.

The sheet passage ability during cold rolling was determined to be lowwhen the risk of trouble during cold rolling increased with increasingrolling load.

The surface characteristics of each cold-rolled steel sheet weredetermined to be poor when defects such as blow hole generation andsegregation on the surface layer of the slab could not be scaled-off,cracks and irregularities on the steel sheet surface increased, and asmooth steel sheet surface could not be obtained. The surfacecharacteristics were also determined to be poor when the amount ofoxides (scales) generated suddenly increased, the interface between thesteel substrate and oxides was roughened, and the surface quality afterpickling and cold rolling degraded, or when some hot-rolling scalesremained after pickling.

Productivity was evaluated according to the lead time costs, including:(1) malformation of a hot-rolled sheet occurred; (2) a hot-rolled sheetrequires straightening before proceeding to the subsequent steps; (3) aprolonged annealing treatment holding time; and (4) a prolongedaustemper holding time (a prolonged holding time at the cooling stoptemperature range in the second annealing treatment). The productivitywas determined to be “high” when none of (1) to (4) applied, “middle”when only (4) applied, and “low” when any of (1) to (3) applied.

It can be seen that the high-strength steel sheets according to exampleseach have a TS of 780 MPa or more, and are each excellent not only inductility, but also in hole expansion formability (stretchflangeability), balance between high strength and ductility, andstability as a material. In contrast, comparative examples are inferiorin terms of one or more of sheet passage ability, productivity,strength, ductility, hole expansion formability (stretch flangeability),balance between strength and ductility, and stability as a material.

1. A high-strength steel sheet comprising: a chemical compositioncontaining, in mass %, C: 0.08% or more and 0.35% or less, Si: 0.50% ormore and 2.50% or less, Mn: 1.60% or more and 3.00% or less, P: 0.001%or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, andN: 0.0005% or more and 0.0100% or less, and optionally at least oneelement selected from the group consisting of Al: 0.01% or more and1.00% or less, Ti: 0.005% or more and 0.100% or less, Nb: 0.005% or moreand 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% ormore and 1.00% or less, Sb: 0.0020% or more and 0.2000% or less, Sn:0.0020% or more and 0.2000% or less, Ta: 0.0010% or more and 0.1000% orless, Ca: 0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and0.0050% or less, and REM: 0.0003% or more and 0.0050% or less, and thebalance consisting of Fe and incidental impurities; a steelmicrostructure that contains, by area, 25% or more and 80% or less offerrite and bainitic ferrite in total, and 3% or more and 20% or less ofmartensite, and that contains, by volume, 10% or more of retainedaustenite, wherein the retained austenite has a mean grain size of 2 μmor less, a mean Mn content in the retained austenite in mass % is atleast 1.2 times the Mn content in the steel sheet in mass %, and an arearatio of retained austenite having a mean C content in mass % at least2.1 times the C content in the steel sheet in mass % is 60% or more ofan area ratio of the entire retained austenite.
 2. (canceled)
 3. Aproduction method for a high-strength steel sheet, the methodcomprising: heating a steel slab having the chemical composition asrecited in claim 1 to 1100° C. or higher and 1300° C. or lower; hotrolling the steel slab with a finisher delivery temperature of 800° C.or higher and 1000° C. or lower to obtain a steel sheet; coiling thesteel sheet at a mean coiling temperature of 450° C. or higher and 700°C. or lower; subjecting the steel sheet to pickling treatment;optionally, retaining the steel sheet at a temperature of 450° C. orhigher and Ac₁ transformation temperature or lower for 900 s or more and36000 s or less; cold rolling the steel sheet at a rolling reduction of30% or more; subjecting the steel sheet to first annealing treatmentwhereby the steel sheet is heated to a temperature of 820° C. or higherand 950° C. or lower; cooling the steel sheet to a first cooling stoptemperature at or below Ms at a mean cooling rate to 500° C. of 15° C./sor higher; subjecting the steel sheet to second annealing treatmentwhereby the steel sheet is reheated to a temperature of 740° C. orhigher and 840° C. or lower; cooling the steel sheet to a temperature ina second cooling stop temperature range of 300° C. to 550° C. at a meancooling rate of 10° C./s or higher and 50° C./s or lower; and retainingthe steel sheet at the second cooling stop temperature range for 10 s ormore, to produce the high-strength steel sheet as recited in claim
 1. 4.The production method for a high-strength steel sheet according to claim3, the method further comprising after the retaining at the secondcooling stop temperature range, subjecting the steel sheet to thirdannealing treatment whereby the steel sheet is heated to a temperatureof 100° C. or higher and 300° C. or lower.
 5. A production method for ahigh-strength galvanized steel sheet, the method comprising subjectingthe high-strength steel sheet as recited in claim 1 to galvanizingtreatment.